linux-kernel - Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core

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Date:	Fri, 15 Jul 2011 17:24:37 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Shan Hai <haishan.bai@...il.com>
Cc:	benh@...nel.crashing.org, paulus@...ba.org, tglx@...utronix.de,
	walken@...gle.com, dhowells@...hat.com, cmetcalf@...era.com,
	tony.luck@...el.com, akpm@...ux-foundation.org,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] Fixup write permission of TLB on powerpc e500 core

On Fri, 2011-07-15 at 11:18 -0400, Shan Hai wrote:

> >> +	vma = find_vma(mm, address);
> > Uhm, find_vma() needs mmap_sem, and futex_atomic_cmpxchg_inatomic() is
> > most certainly not called with that lock held.
> >
> 
> My fault, that will be fixed in the V2 patch.

But you cannot, the function isn't called _atomic_ just for kicks, its
used while holding spinlocks.

> >> +	if (likely(vma)) {
> >> +		/* only fixup present page */
> >> +		if (follow_page(vma, address, FOLL_WRITE)) {
> >> +			handle_mm_fault(mm, vma, address, FAULT_FLAG_WRITE);
> > So how can this toggle your sw dirty/young tracking, that's pretty much
> > what gup(.write=1) does too!
> >
> 
> because of the kernel read only permission of the page is transparent
> to the follow_page(),  the handle_mm_fault() is not to be activated
> in the __get_use_pages(), so the gup(.write=1) could not help to fixup
> the write permission.

So why do you need the vma? Is it like I wrote earlier that you don't
have spare PTE bits and need the vma flags to see if it may become
writable?

gup(.write=1) not triggering this is a serious problem though, not
something you can just paper over. I wouldn't be at all surprised to
find there's more things broken because of that.
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